Conventionally, a zoom lens is designed in accordance with a usual range of wavelengths to be used in imaging an object, and the zoom lens forms an image at a specified image-forming surface at which a detector (e.g., film or detector array) is positioned. However, there has been a problem in that, when a conventional zoom lens is used outside the range of wavelengths for which it was designed, the image no longer is formed at the location of the detector. Therefore, when the zoom lens is used outside the range of wavelengths for which it was designed, the image gets out of focus every time the zoom ratio is changed; thus, refocusing is required.
Some zoom lenses alleviate the inconvenience of having to refocus after each zoom operation when the zoom lens is used at another wavelength range by moving the film plane or the CCD array surface to adjust for the different wavelength range. Other zoom lenses alleviate this inconvenience by inserting a prism between the lens element nearest the image-side and the image-forming surface in order to adjust the optical path length for the change in wavelength range. However, these adjustments make it more difficult to operate the zoom lens.
Japanese Laid-Open Patent Application No. H8-21943 discloses a zoom lens which does not require re-focusing after a zooming operation even when the zoom lens is used outside the range of wavelengths for which it was designed. The zoom lens uses a frame of moving patterns which automatically adjusts the position of certain lens elements in accordance with the wavelength of incident light. Thus, re-focusing is not required every time the zoom ratio is changed despite the zoom lens being used for a different range of wavelengths. However, there is a problem in that the structure of such a zoom lens is complex and two sets of cam grooves are required in order to enable adjustment of certain lens elements that are repositioned to adjust for the change in wavelength. Therefore, further resolution of the problem has been sought.